This application claims the priority of German application 196 24 230.4 filed in Germany on Jun. 18, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a valve timing for an internal-combustion engine operating according to the Otto cycle and having a combination space in a cylinder above a piston, which space is closable by at least one inlet valve and one outlet valve.
Demands on modern internal-combustion engines with respect to reducing emission values as much as possible are constantly rising. In order to meet these demands, emission control systems, particularly catalysts, are as a rule connected behind the internal-combustion engines. These catalysts convert the pollutants contained in the exhaust gas. The problem occurs in this context that naturally the pollutant concentration downstream of the emission control system is the lower the fewer pollutants--so-called raw emissions--exist in the exhaust gas directly behind the internal-combustion engine. On the other hand, there is the problem that, for their efficiency as a result of their construction, emission control systems require a certain operating temperature. If this operating temperature has not been reached, the purification effect is low or non-existent so that, in this operating range, the raw emissions essentially determine the pollutant emissions flowing out of the emission control system.
From German Patent Document DE 35 06 107 A1, it is known to advance the start of the opening of the outlet valve in order to supply to the exhaust system and particularly to the emission control system, a higher-energy, hotter exhaust gas which is to lead to a faster heating-up of the emission control system.
From International Patent Document WO 91/16529, it is known to provide, in addition to a device for advancing the start of the opening of the outlet valve, a device by means of which the opening duration of the inlet valves can be influenced. This device can influence the supplied engine power and thus the exhaust gas temperature.
It is an object of the invention to provide a valve timing for internal-combustion engines by means of which the raw emissions can be reduced.
The invention is based on the recognition that, in contrast to the opinion expressed in the prior art, an early closing of the outlet valve, specifically a closing before the upper dead center is reached, has no influence on the exhaust gas temperature and therefore no influence on the time which the emission control system requires for reaching its operating temperature but, on the contrary, clearly reduces the raw emissions of the internal-combustion engine, here, particularly the hydrocarbon proportions. This effect is particularly pronounced within the range of low loads.
According to the invention, it is therefore suggested to adjust the closing of the outlet valve so far toward early that the outlet valve is closed before the piston reaches the upper dead center. As a result, a defined residual gas volume is retained in the combustion space. Therefore, it is advantageously achieved that mainly the hydrocarbon proportions in the raw emissions but also the nitrogen oxide proportions and carbon monoxide proportions will be reduced.
It is suggested according to an advantageous feature of preferred embodiments of the invention to influence the point in time of the closing of the outlet valve in that the whole timing of the outlet valve is displaced by a predetermined amount in the direction of "early".
As an alternative, it is contemplated according to certain preferred embodiments of the invention to carry out the valve timing such that only the closing of the outlet valve is displaced in the prescribed manner toward "early".
In addition, it is suggested according to certain preferred embodiments of the invention that the above-described measures be carried out only when the load in the internal-combustion engine is low. The limit load up to which the measures are taken may, in addition, be a function of the temperature of the internal-combustion engine in order to expand the load range for the application of the described measures when the internal-combustion engine is cold and the emission control system is therefore also not ready to operate.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.